1. Technical Field
The present invention relates generally to digital filters, and more particularly to a method for detecting and monitoring atmospheric gases, such as gases emitted from smoke stacks or tail pipes, through use of a gas sensor and a digital filter constructed to correlate only with the spectrum of the gas of interest to yield the density of the gas.
2. Discussion
The detection and measurement of trace gases is often essential in order to adhere to environmental standards and regulations, or process control standards. As a result, there is a need by civilian and government agencies for a technique of passively detecting and measuring trace gases in a plume using target plume sensors. The sensors may be as close to an emissions source as the base of a smoke stack, or as far away as on an orbiting satellite.
One problem associated with detecting and measuring trace gases is that the spectral signal of interest associated with the trace gas is typically a small part of the overall signal measured by the sensor. It is often difficult for a basic correlation filter to detect this small target signal unless the background component of the measured spectrum is removed.
Several conventional spectral measurement techniques exist. One technique, known as two spectrum differencing, involves measuring a scene and subtracting the background spectrum from the target spectrum. However, the two pixel differencing does not eliminate spectrally correlated background features in a target spectrum, therefore leading to large errors in gas amount quantification, and leaving a large residual which masks the desired signal.
A second technique, known as model matching, takes a spectral measurement and generates a complex numerical model of the spectrum. The technique then adjusts model parameters until a spectral match is found. However, model matching techniques typically require long, complex computer runs that require a large amount of operator involvement to adjust the model parameters.
A third known technique is known as an orthogonal background suppression (OBS) technique. OBS techniques are used to measure the column density/thermal radiance contrast product of a gas plume using a passive thermal/infrared emission spectrometer. Column density refers to the number of molecules per unit area seen by the sensor, while thermal radiance contrast is the difference between the radiance of the scene behind a plume, and a Planck function generated with the temperature of the plume. Such a technique facilitates target signal detection in the presence of low signal to spectral clutter ratio. OBS techniques are based upon the assumption that background spectral clutter can be assumed to be a linear combination of background spectra taken with no target gas in the field of view. Such a technique finds the proper combination of background scene components and removes the components completely from the target spectrum, thereby leaving only the spectrum of the gas of interest, along with associated random noise.
One advantage of the OBS technique is that an optimal linear filter associated with the technique is able to suppress a large background component of a measurement, while yielding the column density thermal radiance contrast product (DCP) of a gas plume in the atmosphere. However, while present OBS techniques exhibit certain desirable results, the techniques are limited in effectiveness in that the techniques cannot separate the column density measurement from the thermal radiance contrast measurement.
Therefore, there is a need for an OBS based technique of passively detecting and measuring trace gases and a gas plume that allows the value of plume column density to be separated from the thermal radiance contrast.